Air traffic control (ATC) centers are used at most airports to coordinate take-offs, landings, and general aircraft traffic around the airport. Traditionally, a pilot uses a radio to speak to an ATC center to request permission or to receive instructions therefrom. With increasing air traffic it has become difficult for ATC centers to process all of the oral communications from aircraft. Consequently, data-link applications have been developed to provide textual communication between pilots and air traffic controllers.
One of these data-link applications, called Controller Pilot Data Link Communication (CPDLC), provides for the direct exchange of text-based messages between a controller and a pilot. The CPDLC enables the pilot to communicate electronically with an ATC center by guiding the pilot through a series of screen configurations or displays that either elicit flight information from the pilot or notify the pilot regarding flight information. The CPDLC may be part of a larger flight information/control program or may serve as a stand-alone program.
The CPDLC protocol as defined in Eurocae document ED110/RTCA doc 280 requires the pilot to respond to each ground message within 100 seconds. If the pilot needs more time then the pilot has to manually send a “STANDBY” message. The pilot then has 100 more seconds to respond from the time the STANDBY message was sent. If the pilot sends the STANDBY message shortly after receiving the ground message, say 20 seconds, then that does not maximize the amount of time to respond (20 s+100 s=120 s). The pilot can wait and try to send the STANDBY message just before the 100 second time limit expires in order to maximize the time to evaluate the message (e.g., 99 s+100 s=199 s). If the pilot is busy and really needs more time to respond, then the last thing the pilot needs is another task to perform. The air traffic controller is in a similar high pressure situation but with a different time limit of 240 seconds to respond to an aircraft message.